The need for high purity (>99.9%) hydrogen is growing in the chemical process industries, e.g., in steel annealing, silicon manufacturing, hydrogenation of fats and oils, glass making, hydrocracking, methanol production, the production of oxo alcohols, and isomerization processes. This growing demand requires the development of highly efficient separation processes for H2 production from various feed mixtures. In order to obtain highly efficient PSA separation processes, both the capital and operating costs of the PSA system must be reduced.
One way of reducing PSA system cost is to decrease the absorbent inventory and number of beds in the PSA process. In addition, further improvements may be possible using advanced cycles and adsorbents in the PSA process. However, H2 feed gas contains several contaminants, e.g. a feed stream may contain CO2 (20% to 25%) and minor amounts of H2O (<0.5%), CH4 (<3%), CO (<1%) and N2 (<1%). Such a combination of adsorbates at such widely varying compositions presents a significant challenge to efficient adsorbent selection, adsorbent configuration in the adsorber, and the choices of individual adsorbent layers and multiple adsorbent bed systems to obtain an efficient H2-PSA process.
Furthermore, by using fewer valves and faster PSA cycles, i.e., shorter cycle times, lead to significant reduction in adsorbent inventory and PSA system cost. Rotary valves are ideally suited for fast PSA cycles and compact PSA systems. In the application of rotary valves in the PSA systems, the rotary valve devices must accommodate the communication between feed inlet ends and product outlet ends of a PSA system as well as for allowing the flow between beds during pressure equalization step(s) of the process. Pressure equalization normally occurs by transferring a gas from one bed that has just completed its adsorption step to an evacuated bed that has just completed its adsorbent regeneration step.
Relevant background art for PSA systems can be found in the following patents, all of which are hereby incorporated by reference.
U.S. Pat. No. 6,514,319 discloses a gas separation by pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA), to obtain a purified product gas of the less strongly adsorbed fraction of the feed gas mixture, is performed with an apparatus having a plurality of adsorbers. The adsorbers cooperate with the first and second valves in a rotary PSA module, with the PSA cycle characterized by multiple intermediate pressure levels between the higher and lower pressures of the PSA cycle. Gas flows enter or exit the PSA module at the immediate pressure levels as well as the higher and lower pressure levels, under substantially steady conditions of flow and pressure. The PSA module comprises a rotor containing laminated sheet adsorbers and rotating within a stator, with ported valve faces between the rotor and stator to control the timing of the flows entering or exiting the adsorbers in the rotor.
U.S. Pat. No. 6,457,485 discloses a rotary valve assembly for a pressure swing adsorption system. The rotary valve assembly includes a first valve member and a second valve member relatively rotatable about a common center of rotation to provide valving action for selectively transferring fluids therethrough. The second valve member has a first fluid section with at least one aperture adapted for transferring a first fluid of a first pressure and composition therethrough and a second fluid section with at least one aperture adapted for transferring a second fluid of a second pressure and composition therethrough. The first valve member has a first fluid section with at least one passage for transferring the first fluid in the valve assembly and a second fluid section with at least one passage for transferring the second fluid in the valve assembly. A vent is located between the first fluid sections and the second fluid sections of the valve assembly and is vented to a pressure lower than the pressures of the first and second fluids so as to vent leakage from either of the sections of the valve assembly. The rotary valve assembly further includes means for effecting relative rotation of the first valve member and second valve member.
U.S. Pat. No. 5,820,656 discloses a rotary valve system which includes a pair of valve assemblies each of which has valve parts with flat faces which, when pressed together and rotated, provide valving action between various ports incorporated in one valve part of each assembly. The first valve part of each assembly contains a circular array of through openings, each of which is connected to a conduit. The second valve part of each assembly contains several passages which provide communication between various openings of the first valve part and valve apertures located in the second valve part of each assembly. The second valve part of one assembly also contains one or more passages which provide communication between members of one or the other array of openings. The valve system can be effectively used to automate operation of a gas or liquid adsorption system comprising two or more adsorption vessels, the number of vessels being equivalent to the total number of openings in either array. Use of the valve assembly in an adsorption system eliminates the need for many of the valves required in conventional multi vessel adsorption systems.
U.S. Pat. No. 6,143,056 discloses a dual-chambered, four-ported rotary valve with a rotatable drum having peripheral openings is used to selectively connect vacuum, and/or air sources and a vent to adsorption chambers in a two-bed VPSA system to replace separate valves and produce low cost enriched oxygen, via reduced power, increased reliability, and reduced complexity and cost.
U.S. Pat. No. 5,256,171 discloses a pressure-swing adsorption type gas separator system includes two adsorption vessels and a switching valve for placing each of the adsorption vessels alternately in communication with a raw gas source and an exhaust gas line so that a pressurized adsorption process and a reduced-pressure desorption process are sequentially carried out in each of the vessels. The switching valve is a four-port rotary valve having four ports connected to the adsorption vessels, a raw gas source and an exhaust gas line and is driven by a rotary drive. Preferably, the rotary drive is a drive motor for a pressurizing pump provided in a raw gas line, and the four-port rotary valve is connected to the drive motor via a speed reducer.
U.S. Pat. No. 6,503,299 discloses a two bed pressure swing adsorption process for recovering a primary gaseous component at a purity of over 99% from a feed gas comprising the primary component and one or more impurities. Once such process includes: (a) passing the feed gas through a first adsorption bed to remove one or more impurities; (b) conducting a pressure swing adsorption cycle in the first bed; (c) separately passing effluent gases from the first bed into at least two separate tanks for subsequent purging and pressurization of the beds; (d) storing a gas mixture in the first of the tanks containing the primary component in a concentration higher than the concentration of the primary component in the gas mixture in the second of the tanks; (e) refluxing the mixture of the primary component from the second tank in the first adsorption bed during the regeneration steps therein; (f) refluxing the mixture of the primary component from the first tank in the first adsorption bed during the regeneration steps therein; (g) simultaneously and non-concurrently performing steps (a) to (f) in a second bed; and (h) recovering the product gas stream.
U.S. Pat. No. 6,349,382 discloses a pressure swing adsorption process for purifying a syntheses gas stream containing from 60 to 90 mole % hydrogen and impurities such as CO2, CH4, N2, and CO. The PSA process of the invention further provides a method of adsorbing substantially all of the nitrogen and other contaminants from the feed gas stream; wherein the feed stream is passed at superatmospheric pressure through a plurality of adsorbent beds and each adsorbent bed contains at least a CaX, LiA, LiX or calcium containing mixed cation zeolite having a SiO2/Al2/O3 mole ratio of 2.0–2.5. Such process involves sequentially pressurizing, purging and repressurizing the adsorbent beds with product hydrogen, and recovering product hydrogen in purities of 99.9% or greater from the beds.
It is an object of the present invention to provide an indexing rotary dual valve for PSA systems that permits the system to operate with adsorption cycles including pressure equalization step including product gas flow step, vessel purging step and/or product gas to be backfilled.
Another object in the present invention is to provide a low cost H2PSA process using indexing rotary valves for the production of hydrogen from feed gas containing impurities such as CO2, H2O, CH4, CO, N2 and inert gases such as Argon.
Another object of the present invention to provide a H2PSA process which eliminates excess piping connecting the system components.
Another object of the invention is to provide a H2PSA process with improved PSA cycles and efficiencies via the use of indexing rotary valves and control of the individual step times in the PSA cycle.
Another object of the invention is to provide new PSA cycles; wherein, variable bed inlet flow rate is utilized to control the spreading of the mass transfer zones; thereby, achieving enhanced PSA process performance over prior art H2PSA processes.